Outside of having to have a mask, snorkel and usually fins it doesn't cost anything. Now you will still need to get transportation to the area so whatever that costs and if you do it as a tour the people putting on the tour need to make money and that all depends on the location etc. In other words, for a tour it varies greatly from one port to another.
Re: Is snorkeling rental equipment readily available? Price
Again Don, it depends on where you are at. In Cozumel you can rent it right there at the Lagoon for about $10 for mask, fins, and snorkel and vest. In GC you can rent them at Edans Rock and Devils Grotto again very cheap. In more remote areas you need to bring it with you and the ship doesn't rent except to those on their excursions AFAIK. You may wish to conside buying your own and brining it with you. Lots cleaner that way as well. <G>